Hard disks drives have become ubiquitous for high volume, non-volatile storage of electronic data. While principally used as data storage devices for computing systems, hard drives have found additional uses, including, for example, in video and audio recording systems, and in small, highly portable music playback systems. As with many types of electronic devices, very substantial efforts have been made over recent years to increase the performance of hard disk drives. These efforts have primarily been directed to increasing hard disk storage capacity, reliability and robustness, while reducing cost, size, and data access (read/write) times.
In hard disk drives, data is stored on a spinning hard disk or platter, using a recording head, in digital form, as a series of binary bits, each of which is stored at a precise, known, physical location on a surface of the disk. Typically, modern hard disk drives comprise multiple, coaxial, stacked platters, each of which comprises an aluminum or other substrate having a magnetic film deposited on both the upper and lower surfaces of the platter. As is well known in the art, data is stored by the polarization of the magnetic domains in small, well-defined areas of the magnetic film on the platter. The magnetic domains are oriented using a disk drive write head comprising a coil used to transmit a precise electromagnetic signal to orient the magnetization of a domain on the surface of the disk immediately adjacent to the head. In this manner, the magnetic field at the surface of the disk at a given location is made to represent either a logical 1 or 0, corresponding to the desired binary bit value, and can thereafter be read back (or changed) using the read/write head. In practical terms, as is well known, the data is actually stored in the form of magnetic transitions from one domain to the next.
It can be appreciated that the amount of data that can be stored on a hard disk drive is a function of both the overall available area on the disk surface and the area required to store each bit (including the area necessary to separate adjacent bit storage locations). For practical reasons, the size of the disks has actually been decreasing. Accordingly, in order to increase storage capacity, great attention has been paid to reducing the already very small area on the surface of the disk necessary to store data bits. Important factors in this effort include reducing the separation distance between the read/write head and the disk surface (the “flying height”), improving the uniformity of the magnetic film, and reducing the size of the domains so that very small areas on the usable surface of a disk can be reliably used for data storage. However, as the areal extent of the magnetic surface used for storage of a data bit is decreased, any small defects or imperfections in the area take on greater significance.
In traditional prior art hard disk drives the magnetic domains 10 are horizontally or “longitudinally” aligned on the surface of the magnetic film as depicted in FIG. 1A. Reversing the magnetization of a domain relative to the adjacent domains, causes a magnetic transition 20 which is detectable when a read/write head passes over the transition area and detects a variation in the magnetic flux above the surface of the magnetic film. However, there are practical limits to the size of horizontal magnetic domains. Specifically, after a limit is reached, smaller magnetic domains are inherently unstable due to thermal fluctuations.
New generations of hard disk drives use vertically polarized magnetic domains 30 to reduce the amount of space needed to store data, as shown in FIG. 1B. Again, magnetic transitions 40 between adjacent domains can be created using a write head and, thereafter, detected using a read head. One estimate is that vertical magnetic polarization, or “perpendicular” data storage, can increase the storage capacity of a disk ten-fold. However, vertical drives use thicker magnetic films and require a “soft” magnetically permeable underlayer 50, which can increase the manufacturing difficulty of achieving a highly uniform, planar surface.
Normally, the disk surface may be viewed as comprising a plurality of contiguous annular regions or “tracks” that are used for data storage. Track widths of vertical hard disks are of the order of 100 nanometers, and track density is of the order of 2,400,000 transitions per inch.
As can be understood from FIG. 1B, when all of the magnetic domains of a vertical disk are aligned in the same direction, i.e., when there are no transitions, the entire surface has a single magnetic polarity and the magnetic field adjacent to the surface of the disk is substantially uniform. In contrast, when there are no transitions in a horizontal disk, the magnetic field varies with location.
After manufacture, the platters of a hard disk drive need to be tested for defects and to ensure that they meet specifications. Testing is typically performed on unformatted disk platters prior to final disk drive assembly. For the reasons discussed above, the specifications are becoming more stringent as smaller disk areas are used for data storage. Small scratches, pits and other defects in the surface of the magnetic film are particularly critical and the existence of any such defects needs to be identified. By identifying the location of spatial defects, data loss is avoided by marking the area as defective prior to use, or by discarding the disk entirely if it is found to have too many defects. It is noted that vertical domains can be smaller than optical detection limits, such that optical inspection of the disk surface cannot be used to identify surface irregularities that impact device performance.
In addition to spatial defects such as scratches or other irregularities in the disk surface, hard disks are also subject to “thermal” defects that may occur along with or separately from the spatial defects. This type of defect is essentially a small bump or protrusion on the surface of the platter, where the height of the bump is such that the read head makes contact with the bump, but is able to continue scanning the surface of the disk (i.e., the bump is not so large that the read head stops functioning). When the read head encounters the bump, the high speed impact causes the read head to increase in temperature (hence the name “thermal” defect). Repeated impacts lead to wear on the read head and can eventually cause the head to “crash” into the surface of the hard disk. As such, thermal defects on a hard disk pose an even greater problem than spatial ones. While spatial defects may limit the amount of disk space available to store data, thermal defects may cause the hard disk to crash, such that data on the disk may or may not be recoverable. For this reason, a disk having predominantly spatial defects and few thermal ones will be more usable than a disk having the same total number of defects, but where a substantial number of the defects are thermal. Accordingly, there is a need for a system and method to identify spatial and thermal defects on a vertical hard disk platter and to distinguish each type of defect from the other.
A common testing technique currently in use is referred to as the “missing pulse test.” The missing pulse test involves writing a sinusoidal waveform to the surface of the disk using a write head, and then reading back the recorded signal using a read head. Because a sine wave has two transitions per cycle, the read back frequency is twice the write frequency. Discrepancies, referred to as “dropouts”, between what is written and what is read are used to identify disk errors. As domain size has decreased, it has become necessary to use higher frequency to properly analyze the surface of a disk. Currently, write frequencies as high as 200 MHz (and corresponding read frequencies of 400 MHz) may be used.